Somehow the question made me think he wanted something off the shelf.
You used to be able to get adapters that would flash lights that were plugged in. Not much more than a pair of prongs on one side, and an outlet on the other, and you'd plug your Christmas tree lights into that. I gather something would heat up, which would then cause a contact to open, at which point it would cool down and the contact would close again, causing power to flow and not flow to the outlet. COmpletely mechanical.
So you get one of those, then an AC adapter that puts out the needed DC voltage. Plug the flasher into the wall, plug the AC adapter into the flasher, then connect the AC adapter's output to the string of bulbs, making sure polarity is correct if it needs to be.
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